Field of the Invention
Energy costs, particularly as illustrated by the cost of crude oil and liquid petroleum distillates derived from crude oil have escalated rapidly. These costs are especially burdensome to the users of transportion fuels, such as shipowners, and these major consumers of petroleum products are reacting to these events and are searching for more efficient measures to use in their operations. One significant development in the shipping field is the trend away from steam turbine propulsion units in favor of large marine diesel engines which are more fuel efficient with respect to petroleum fuels.
The largest marine diesel engines used for ship propulsion are classified as slow speed marine diesel engines. These engines are unique both in their size and in their method of operation. The engines themselves are massive, the larger units approaching 2000 tons in weight and upwards of 100 feet long and 45 feet in height. Their output can reach 50,000 brake horsepower with engine revolutions ranging from about 100 to 125 revolutions per minute.
The slow speed marine diesel engines are also unique in their design. Most notably, the crankcase of the large slow speed single acting 2-stroke crosshead type of engine is completely separate from the combustion chambers of the engine. Because of this, its lubrication requirement differs from that of a typical diesel engine. In particular, the upper cylinder portion of the slow speed diesel engine, not being in direct communication with the crankcase zone of the engine, has it own lubrication system with specific lubrication requirements that differ markedly from the requirements of a crankcase lubricant. In addition, and for reasons of economy, the fuel employed to run the large slow speed diesel engines are residual fuels having relatively high levels of sulfur. This circumstance dictates the employment of a highly overbased lubricating oil composition in order to counteract the acidity generated during the combustion of the sulfur-containing fuel. As a result, a typical cylinder lubricating oil composition for a slow speed marine diesel engine will have an alkalinity level expressed as total base number ranging between about 50 and 100.
The fuel consumption rate of a marine diesel engine of 30,000 horsepower ranges upwards of 1200 gallons of fuel per hour. In view of the current need to reduce overall oil consumption, intensive efforts are being made to discover lubricating oil compositions which can materially reduce the friction losses which take place within the engine itself. Reductions in engine friction losses translate directly into significant fuel savings.
Numerous means have been employed to reduce the friction in internal combustion engines. These range from the use of lower viscosity lubricating oils or mixtures of mineral and synthetic lubricating oils as well as to the incorporation of friction-reducing additives such as graphite, molybdenum compounds and other chemical additives. There are limits to the extent to which the viscosity of a lubricating oil can be reduced for the purpose of reducing friction. Generally, a lubricating oil having too light a viscosity will fail to prevent metal-to-metal contact during high load operating conditions with the result that unacceptable wear will occur in the engine. With respect to chemical anti-friction additives, significant research efforts are ongoing to find effective and economic antifriction additives which exhibit stability over an extended service life and under a wide range of operating conditions.
It is an object of this invention to provide a novel cylinder lubricating oil composition for a slow speed marine diesel engine.
It is another object of this invention to provide a highly overbased marine cyliner lubricating oil having improved friction properties for lubricating a slow speed marine diesel engine.